1. Field of the Invention
This invention relates to the art or process of heat treating metal parts or workpieces for industrial applications using the well known isothermal transformation technique resulting in improved physical and chemical characteristics of the workpieces and is particularly applicable to such a process which is automated or semi-automated for handling metal parts or workpieces of various size, shape, mass, geometry, chemical composition and thermo-property.
2. Description of the Prior Art
Heat treating of metals to achieve desired chemical and physical properties, in many cases, requires the isothermal decomposition of metallurgical constituents. This process is achieved by first heating the metal above the solution treating transformation or austenitizing temperature A.sub.1 where, given enough time, a homogeneous solid solution is formed. In the case of steel it is seen from the iron-cementite phase of the Isothermal Transformation Diagram that an eutectoid composition transforms completely, in time, to austenite at 1333.degree. F. In normal heat treating practice, to be sure all sections of each workpiece and all workpieces are above the A.sub.1 temperature, the workpieces will be heated to a temperature above the A.sub.1 while eutectoid steel will be heated to around 1500.degree. F. After achieving solid solution by heating the workpiece to a temperature above the A.sub.1 temperature for the proper amount of time, the solid solution is converted by cooling to the desired mixed solids. In many cases to achieve the desired mixed solids and to avoid the formation of other mixtures or undesirable proportions of desired mixed solids, it is necessary to super-cool the workpiece from above the A.sub.1 temperature. The practice of transporting the workpiece from the heat source or station to the cooling source or station is now being done mechanically as is disclosed and described in the Upton et al U.S. Pat. No. 3,459,313 entitled "Work Transporting Apparatus", assigned to the assignee of record.
In certain heat treating processes, it may be necessary to transfer a workpiece from a furnace to a liquid quench in a matter of seconds, as for example, from one to five seconds. It is difficult to utilize conveyor structures of the mechanical type using hook-type hanger elements for support since the workpieces supported on the hangers would swing and splash when lowered into the liquid quench tank. Swinging is further objectionable because it may cause the workpiece to become disengaged from the conveyor and may also result in the distortion of the workpiece. Even though the mechanical rotary transfer arm apparatus disclosed in U.S. Pat. No. 3,459,313 was an improvement over the hook-type hanger elements just described by providing a transfer device which minimized the swinging of workpieces and permitted the transfer of workpieces from one station to another more rapidly than prior art devices, it nevertheless cannot physically handle various workpieces of certain mass, geometry, size and shape and transport such workpieces from the heating station to the cooling station in the time period available which may be as short as one to five seconds. Thus with the use of mechanical transfer devices, time is of the essence for transferring the workpiece from the heating station to the cooling station.
In the case of molten salt bath heat treating or pit type operations, as an example, using the transfer device disclosed in U.S. Pat. No. 3,459,313, the workpiece is exposed to the cooling effects of the atmosphere which exists between the heating source and the quench or cooling source. To avoid excess cooling of the workpiece during transport, conventional apparatuses, employing speed of movement alone, are used which quickly transport the workpiece from one station to another. In many cases, the mass, geometry and/or chemical composition of the workpiece requires a transport time less than mechanically possible by conventional means, to avoid cooling below the A.sub.1 temperature thus, limiting the practical use of the isothermal transformation process to those metals whose mass, geometry and/or chemical composition permits transport in the times required by conventional mechanical devices. Conventional mechanical devices thus do not provide a means by which heat is kept from leaving the workpiece, thereby making it possible for other masses, geometries and/or chemical compositions to have temperatures below the A.sub.1 temperature before entering the quench or cooling station. This is undesirable since the desired physical and chemical properties of the workpieces are not obtained resulting in scrap or inferior products.
Various devices have been disclosed for heat treating parts by enclosing the workpiece in a shield; however such shield has not been used for increasing the versatility of isothermal transformation of metal workpieces as disclosed herein. A preliminary novelty search resulted in the following United States patents: U.S. Pat. No. 2,142,139 of A. W. Machlet, dated Jan. 3, 1939; U.S. Pat. No. 2,417,610 of H. P. Phillips, dated Mar. 18, 1947; U.S. Pat. No. 2,862,843 of W. E. Engelhard, dated Dec. 2, 1958; U.S. Pat. No. 3,415,694 of W. E. Engelhard, dated Dec. 10, 1968; and U.S. Pat. No. 3,972,751, of W. E. Engelhard, dated Aug. 3, 1976. However, none of the prior art devices or methods are directed to preventing the temperature of the metal workpieces from falling below the A.sub.1 or solution treating transformation temperature during transportation of the workpieces from the heating station to the cooling station or to products which have improved metallurgical and physical properties achieved by processes like the austempering and mar-quenching techniques and are obtained economically, thus eliminating waste and scrap.